[Ada] Simplify returning from Enclosing_Comp_Unit_Node

[Pierre-Marie de Rodat]

Code cleanup; semantics is unaffected.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * sem_util.adb (Enclosing_Comp_Unit_Node): When the loop exits
        the Current_Node is either an N_Compilation_Unit node or Empty,
        so simply return it without redundant checks.
diff --git a/gcc/ada/sem_util.adb b/gcc/ada/sem_util.adb
--- a/gcc/ada/sem_util.adb
+++ b/gcc/ada/sem_util.adb
@@ -8037,11 +8037,7 @@ package body Sem_Util is
          Current_Node := Parent (Current_Node);
       end loop;
 
-      if Nkind (Current_Node) /= N_Compilation_Unit then
-         return Empty;
-      else
-         return Current_Node;
-      end if;
+      return Current_Node;
    end Enclosing_Comp_Unit_Node;
 
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